Compression-type refrigerating machines have conventionally employed chlorofluorocarbon refrigerants such as CFC-11 (CCl.sub.3 F, trichloromonofluoromethane), CFC-12 (CCl.sub.2 F.sub.2, dichlorodifluoromethane), HCFC-22 (CHClF.sub.2, monochlorodifluoromethane), and CFC-115 (CF.sub.3 CClF.sub.2, monochloropentafluoroethane). However, the use of chlorofluorocarbons including CFC-12 has been restricted since they cause ozone layer depletion. Although HCFC-22 has not been restricted in its use so far because of its less ability to deplete the ozone layer, the use thereof will be restricted in the future.
As substitutes for these chlorofluorocarbons, chlorine-free hydrofluorocarbons are coming to be used. Proposed as a substitute for CFC-12 is HFC-134a (CH.sub.2 FCF.sub.3, 1,1,1,2-tetrafluoroethane), which is similar in thermodynamic properties to CFC-12. Proposed as a substitute for HCFC-22 is a mixed refrigerant which contains HFC-32 (CH.sub.2 F.sub.2, difluoromethane) and is similar in thermodynamic properties to HCFC-22.
A refrigerating machine oil is required to have various performances, of which the compatibility with a refrigerant is extremely important from the standpoints of the lubricity of the oil and the efficiency of the system. It is, however, known that chlorine-free hydrofluorocarbon refrigerants represented by HFC-134a and HFC-32 are almost incompatible with the refrigerating machine oils conventionally used in compression-type refrigerating systems, which oils contain a naphthene-based mineral oil, paraffin-based mineral oil, alkylbenzene, or the like as the base oil, and that the working fluids containing such chlorine-free hydrofluorocarbon refrigerants undergo two-phase separation both in a low-temperature side and in a high-temperature side.
If the two-phase separation occurs, the lubricating oil is retained in the condenser and expansion device, resulting in a decrease of the efficiency of refrigeration and in insufficient supply of the lubricating oil to the slide way in the compressor. Since the defective lubrication causes troubles including seizure of the compressor, the refrigerating machine cannot be applicable to practical use.
Under these circumstances, various lubricating oils compatible with chlorine-free hydrofluorocarbon refrigerants have been proposed. For example, U.S. Pat. No. 4,755,316 proposes a lubricating oil based on a polyoxyalkylene glycol having a specific molecular weight distribution and terminated by a hydroxyl group at both ends. Although this lubricating oil is compatible with HFC-134a in the temperature range of from about -40.degree. C. to +50.degree. C., the compatibility at higher temperatures is necessary for practical use.
On the other hand, HFC-134a is used mainly in home refrigerators and automotive air-conditioners, and mixed refrigerants containing HFC-32 are goint to be used mainly in home air-conditioners and industrial refrigerating machines. Home refrigerators and home air-conditioners are mostly of the type in which the motor for driving the compressor is used in a refrigerant-refrigerating machine oil mixture and, hence, the refrigerating machine oil is required to have excellent electric insulating property. However, the polyoxyalkylene glycol has much poorer electric insulating property than the conventional naphthene-based mineral oil and paraffin-based mineral oil and also has high hygroscopicity. Consequently, the polyoxyalkylene glycol is unsuitable for use as a refrigerating machine oil for home refrigerators or home air-conditioners.
In WO 90-12849, a polyol ester obtained from a monocarboxylic acid and a polyhydric alcohol and a complex ester obtained from a monocarboxylic acid, a polycarboxylic acid, and a polyhydric alcohol are proposed as lubricating oils for use with a chlorine-free hydrofluorocarbon refrigerant.
Further, as other lubricating oils for use with a chlorine-free hydrofluorocarbon refrigerant, a polyol ester and a complex ester each derived from a condensate of a monohydroxycarboxylic acid with a dihydric neopentyl polyol and from a mono- or dicarboxylic acid are proposed in the 41st K obunshi T oron-kai (September, 1992; sponsored by the Society of Polymer Science, Japan; Polymer Preprints, Japan, Vol.41, No.11, pp. 4703-4705).
These proposed esters have lower hygroscopicity than the polyoxyalkylene glycol and are well compatible with HFC-134a in a wider temperature range than the polyoxyalkylene glycol. The esters also have good electric insulating property, with their volume resistivities being about 10.sup.13 to 10.sup.14 .OMEGA.cm at 80.degree. C., as described in EP 406,479-A1; such resistivity values suffice for refrigerating machine oils for use in refrigerators, home air-conditioners, or the like.
Refrigerating machine oils are also required to be supplied in various viscosity grades according to the kinds of refrigerating machines, etc., and the oils currently in use are mostly of ISO viscosity grades VG8 to VG320. The complex esters can provide esters which have good electric insulating property and are of various viscosity grades.
However, since the above-described esters proposed in the art are subject to hydrolysis in the presence of water, there is a fear of corroding the refrigerating system. The polyol esters can inhibit hydrolysis to a practically acceptable level by employing a branched fatty acid as the monocarboxylic acid as one of the starting materials. Although being satisfactory in lubricity and in compatibility with HGC-134a, the complex esters are inferior in hydrolytic stability to the polyol esters. The poor hydrolytic stability of the complex esters may be attributable to the fact that most of the commercially available polycarboxylic acids are linear; the bonded units derived from a linear polycarboxylic acid are liable to hydrolyze.
The polyol esters and complex esters proposed so far are also defective in that the compatibility thereof with a mixed refrigerant containing HFC-32 is still insufficient, although they are compatible with HFC-134a almost satisfactorily.
Furthermore, the esters proposed in the 41st K obunshi T oron-kai, for which a monohydroxycarboxylic acid was used as one of the starting materials, have the following drawbacks. The proposed compounds have a molecular structure comprising units of the ester of a dihydric alcohol with the monohydroxycarboxylic acid and, in order to obtain a high-viscosity ester, these units are bridged with a dicarboxylic acid to give a complex ester. As a result, such high-viscosity esters have poor hydrolytic stability like other complex esters. When the bridging with a dicarboxylic acid is absent, it is difficult to obtain a high-viscosity ester having good compatibility with chlorine-free hydrofluorocarbon refrigerants.